An article by W. T. Dixon vol. 153 pp. 189-194 appearing in the journal Radiology entitled "Simple Proton Spectroscopic Imaging" gave the initial impetus to the procedures used for the separation of spectral components in MRI imaging. That article explained the method of encoding spectroscopic information into clinical images. The initial Dixon method has subsequently been improved upon as explained in a plethora of patents and articles. Some of the patents are listed in the above mentioned patent.
The images differentiating between water and fat intensities in the basic Dixon method of the prior art were produced using a spin echo sequence in which the spin echo and a gradient echo coincide. In addition, each excitation was repeated with the spin echo shifted by an appropriate interval. More particularly the 180.degree. RF pulse was shifted by a time T to shift the spin echo with respect to the gradient echo an amount sufficient to cause the chemical shift between the echoes of water and lipids to be 180.degree. out of phase at the gradient echo time. The image produced with the described sequence clearly indicates the differences between the signals due to water and the signals due to fat.
A major problem with the original Dixon method is its susceptibility to main field inhomogeneity. In the past, several improved water and fat separated imaging methods have been developed to obtain water images and fat images in one scan. They can be classified in two major categories:
1. The two point method of the previously mentioned patent, and PA0 2. The general three point method developed by Qing-San Xiang et al published in ISMRM pg. 1544 (1996) entitled "General Three Point Water Fat Imaging with Optimized SNR", the disclosure of which is incorporated herein by reference.
The data acquisition time of the general three point method is about three times longer than the normal scan. The data processing time of the general three point method is on the order of 10-20 seconds for a 256.times.256 pixel image. The data acquisition time of the two point Dixon method disclosed in the above mentioned patent is only two times longer than a normal scan and the data processing time is 1-2 seconds for a 256.times.256 pixel image.
The improvements of two times versus three times the scan time and the reduction of the processing time from 10-20 to 1-2 seconds per image clearly is a vast improvement that makes the two point Dixon method the method of choice for clinical use. However, until now the two point Dixon method had limitations which proscribed its use in certain situations.
For example, it has not been used for phased array coil images. Phased array coils comprise a plurality of individual coils herein referred to as "segments". With such arrangements it is necessary to separate the water and fat images obtained from each of the different segments of the phased array coil arrangement and to separately combine all of the water images of each of the segments and to separately combine all of the fat images of each of the segments.
It is known that in addition to the images, the phase distortion caused by inhomogeneity of the main field Bo can be determined from the two scans that came from the original Dixon method. See for example the article by Coombs, BD et al in SMRM, pg. 647 (1995), the disclosure of which is incorporated herein by reference.
In addition to the inhomogeneity problem, another problem solved by the method of the above mentioned patent application is the required phase unwrapping. Unwrapping methods prior to the above mentioned patent were tedious, non-robust and unreliable when phase wrapping was present which was most of the time. The above mentioned patent presents a method of overcoming the phase unwrapping problem.
An improvement provided by some preferred embodiments of the invention herein is, among other things, to use the method of the patent successfully with phased array coil arrangements and more particularly to provide a system that separately combines the water images from the different segments and the fat images from the different segments of a phased array coil arrangement speedily and reliably and/or which is immune to field inhomogeneity for practical purposes.
In spectral imaging using whole body coils there is really no need to determine which of the two images obtained is a fat image and which is a water image. The person doing the scan can easily determine this based on the appearance of the image. However, when using phased array coils it is necessary to combine all of the images of one spectral component separately and all of the images of the other spectral component separately. Thus while it is not necessary to determine which data comprise the fat images and which data comprise the water images; it is necessary to keep the two types of data separated. Like data are then combined to form the two spectral images.
The two point Dixon method described in the above named patent did not match the data; instead it used a histogram method for differentiating between fat images and water images. Since it was not necessary to actually determine which images are fat and which images are water in the two point Dixon method of the patent, it was not necessary to use any method for determining which are the fat and which are the water images. Rather it was necessary to determine which data match together, i.e. are either from a first or a second spectral component.
Accordingly, a broad aspect of some preferred embodiments of the present invention is to provide a method and system for separating spectral images during the production of the different spectral images while utilizing a phased array coil arrangement for obtaining the image data.
Accordingly, it is another object of some preferred embodiments of the present invention to acquire separate data contributions from first and second spectral components, for correcting the data for the phase wrapping that is due to the inhomogeneity of fields within the sample, as well as due to linear phase delays in a robust, reliable, repeatable and relatively simple manner.